The present invention relates to a photomask blank and a photomask for use in the microfabrication of semiconductor integrated circuits and high density integrated circuits. It also relates to a method of manufacturing the same.
Photolithography using photomasks is employed for the microfabrication of high density semiconductor integrated circuits such as LSI and VLSI, color filters for charge coupled devices (CCD) and liquid crystal displays (LCD), magnetic heads, and the like. The photomasks are prepared from photomask blanks.
The commonly used photomask blank has a light-shielding film formed on a transparent substrate of quartz glass or the like. The light-shielding film is most often in the form of a chromium base film and prepared by such techniques as sputtering and vacuum evaporation.
Photomask blanks of multilayer structure are also known wherein a chromium base film serving as a light-shielding film is formed on its surface side with CrO for preventing light reflection or on both its surface and substrate sides with antireflective films of CrO.
The photomasks of multilayer structure have the advantage of a broad exposure latitude permissible upon pattern imprinting on a wafer. However, they are difficult to produce a mask having good image quality and durability by forming an image on the photomask blank, as compared with the single layer type chromium mask.
More particularly, when the photomask blank of multilayer structure is etched and patterned by conventional lithography, steps are formed in cross-sectional shape between the chromium base light-shielding film and the chromium base antireflective film because of their different etch rates.
On the other hand, the etch rate largely changes midway the film with a likelihood that the film as a whole be unevenly etched. Uneven etching can detract from the sharpness of the image and leave overhangs around the pattern.
To solve these problems, JP-B 62-27386 discloses a photomask blank comprising a nitrogen-containing chromium base light-shielding layer having a continuously changing degree of nitriding; JP-B 62-37386 discloses a photomask blank comprising a carbon-containing chromium base light-shielding layer having a continuously changing degree of carbonization; and JP-B 61-46821 discloses a photomask blank having a changing compositional ratio of chromium oxide to chromium nitride. And JP-A 4-9847 discloses a photomask blank comprising an antireflective film formed of chromium carbide, chromium nitride and chromium oxide in which the degree of oxidation continuously changes.
As mentioned above, a photomask is prepared by starting with a conventional photomask blank and selectively removing the light-shielding film and the antireflective film, whereby the resulting photomask has a low reflective, light-shielding film pattern consisting of the patterned light-shielding film and the patterned antireflective film. A side etch rate can be controlled by taking an appropriate approach when the light-shielding film and the antireflective film are deposited. Such approaches include the addition of oxygen to chromium, formation of oxygen-containing chromium on the light-shielding film, and introduction of oxygen gas into a deposition chamber as the oxygen source. The use of oxygen gas as the oxygen source, however, gives rise to the problem that optical properties such as transmittance, reflectance and refractive index become non-uniform within the substrate plane.
An object of the invention is to provide a photomask blank and a photomask of quality which have uniform film properties and avoid any degradation of cross-sectional shape caused by differential etch rate between the light-shielding film and the antireflective film. Another object of the invention is to provide methods for preparing the photomask blank and the photomask.
It has been found that when chromium oxynitride carbide is deposited on a transparent substrate by effecting reactive sputtering, the uniformity of optical properties within the substrate plane is improved by using a target of chromium or chromium containing at least one element of oxygen, nitrogen and carbon and a sputtering gas containing at least carbon dioxide gas, a nitrogen-containing gas and an inert gas. This enables controlled deposition of the chromium base film and mass scale production in a consistent manner, ensuring that a high quality chromium base film of chromium oxynitride carbide is obtained. At the same time, the concentration of carbon atoms in the chromium oxynitride carbide is reduced stepwise or continuously from the surface toward the substrate, which enables to control an etch rate, successfully overcoming the above-mentioned problems.
In one aspect, the invention provides a photomask blank comprising a transparent substrate, at least one layer of light-shielding film and at least one layer of antireflective film both on the substrate, wherein the light-shielding film and the antireflective film are formed of a chromium base material containing oxygen, nitrogen and carbon such that the content of carbon decreases stepwise or continuously from a surface side toward the substrate. Preferably, the light-shielding film and the antireflective film each are formed of a chromium oxynitride carbide.
A photomask fabricated by lithographically patterning the photomask blank is also contemplated herein.
In another aspect, the invention provides a method of manufacturing a photomask blank comprising a transparent substrate, at least one layer of light-shielding film and at least one layer of antireflective film both on the substrate. The method involves the step of forming the light-shielding film and the antireflective film by effecting reactive sputtering using a target of chromium or chromium containing at least one element of oxygen, nitrogen and carbon and a sputtering gas containing at least carbon dioxide gas, a nitrogen-containing gas and an inert gas, so that the content of carbon decreases stepwise or continuously from a surface side toward the substrate. Preferably, the light-shielding film and the antireflective film each are formed of a chromium oxynitride carbide. The reactive sputtering step may include changing the proportion of the carbon dioxide gas in the sputtering gas for controlling the content of carbon in chromium oxynitride carbide of the light-shielding film and the antireflective film.
In a further aspect, the invention provides a method of manufacturing a photomask, comprising the step of lithographically patterning the photomask blank manufactured by the above method.
When a chromium base oxide film is deposited on a transparent substrate by a reactive sputtering technique, the invention recommends to use carbon dioxide gas as the oxygen source. Since carbon dioxide is less reactive than oxygen, the gas can uniformly spread and distribute in the chamber over its full extent so that the chromium base film being deposited becomes homogeneous. Furthermore, the content of carbon in the chromium base film is reduced from the surface side toward the transparent substrate, thereby enabling the photomask to be given a pattern having a cross-sectional shape perpendicular to the substrate and free of steps. The photomask thus contributes to the microfabrication of semiconductor ICs of greater density and finer feature size.
The film deposition by sputtering using carbon dioxide gas as the oxygen source can minimize the non-uniformity of optical properties within the substrate plane for the following reason. In the process of flowing oxygen or another reactive gas into a deposition chamber from its inlet, oxygen is successively consumed for oxidation from a position nearer to the inlet, that is, the degree of oxidation is higher at a position nearer to the inlet. When oxygen is fed from without the target, oxygen is consumed on the outside of the target so that the concentration of oxygen lowers toward the inside. As a result, the degree of oxidation increases from the center of the chamber toward the outside, producing a distribution of reflectance and optical constants.
In contrast, the invention uses low reactivity carbon dioxide gas as the oxygen source in reactive sputtering. Since the carbon dioxide gas is little consumed until it is activated by a plasma, it can uniformly spread throughout the chamber so that a film being deposited may have a more uniform degree of oxidation. As a result, the distribution of optical properties within the substrate plane is drastically improved. Film properties can be controlled by using both carbon dioxide gas and an inert gas as the sputtering gas and adjusting the mixing ratio therebetween.
Further, the content of carbon in chromium oxynitride carbide is reduced stepwise or continuously from the surface toward the transparent substrate, so that there are available a slow etch rate on the surface side and a fast etch rate on the substrate side. The resulting pattern has a cross-sectional shape perpendicular to the substrate and having no steps. The precision of pattern transfer is dramatically improved. The carbon content can be easily adjusted by stepwise or continuously changing the mixing ratio of inert gas and carbon dioxide gas in the sputtering gas.